1. Field of the Invention
The present invention relates to multiband telecommunication antennas, in particular for cellular telephone systems.
2. Description of the Prior Art
Cellular telephone systems use various frequency bands corresponding to various existing telecommunication systems. Several telecommunication systems are used simultaneously at present, for example the digital cellular system (DCS) (1 710-1 880 MHz), and the Global System for Mobile communications (GSM) (870-960 MHz). New telecommunication systems are currently being installed, such as the Universal Mobile Telephone Service (UMTS) (1 900-2 170 MHz).
Telecommunication network operators must therefore provide a network of antennas operating in the various frequency bands used. Some operators install complementary networks of antennas, each network operating in accordance with one telecommunication system. Thus operators use a network of GSM antennas and a network of DCS antennas while they are installing a network of UMTS antennas.
However, the multiplication of antenna networks leads to increasing costs for the operatorsxe2x80x94purchase of antennas, leasing of locations, installationxe2x80x94and damages the environment. For this reason other operators use antennas operating in accordance with more than one telecommunications system. This reduces the installation cost and damage to the environment.
Two types of antennas are then used:
A first type of antenna, known as a xe2x80x9cwidebandxe2x80x9d antenna, uses a sufficiently wide operating band to be able to send and receive calls in accordance with more than one telecommunication system. For example, an antenna using a frequency band from 870 MHz to 1 880 MHz is used as a combined GSM and DCS antenna.
A second type of antenna, known as a xe2x80x9cmultibandxe2x80x9d antenna, combines, in a single antenna chassis, respective radiating elements conforming to more than one telecommunication system. For example, there are GSM and DCS dual band antennas including respective radiating elements for the GSM and the DCS.
FIG. 1 shows a prior art GSM and DCS dual band antenna. The dual band antenna 10 includes radiating elements 12 operating in accordance with the GSM and radiating elements 14 operating in accordance with the DCS. In this kind of antenna the GSM radiating elements 12 are connected to two GSM connectors 16 and 18 transmitting waves with frequencies in the GSM band. Similarly, the DCS radiating elements 14 are connected to two DCS connectors 20 and 22 transmitting waves with frequencies in the DCS band. FIG. 1 does not show the connection between the connectors and the GSM or DCS radiating elements.
Two independent connectors transmitting waves in the same frequency band are used because of the nature of the radiating elements used. Each radiating elementxe2x80x94the operation of which is described in U.S. Pat. No. 6,025,798, for examplexe2x80x94is equivalent to two independent dipoles at 90xc2x0 to each other. Accordingly, the radiating elements 12 and 14 receive and/or send telecommunication signals correctly regardless of the position of a sending or receiving antenna relative to the radiating elements.
The set of radiating elements for the same band of frequencies forms a transmission device. Accordingly, the GSM radiating elements 12 form a GSM transmission device and the DCS radiating elements 14 form a DCS transmission device. To optimize the operation of each of these devices, two criteria are taken into account in the design of this prior art antenna:
In accordance with a first criterion, the radiating elements for the same band of frequencies are separated by a distance substantially equal to 0.95xc3x97xcexm, where xcexm represents the average wavelength of the band of frequencies associated with those radiating elements. It is known that this disposition of the radiating elements is favorable to the operation of the device positioned in this way.
In accordance with a second criterion, the radiating elements of the same device are placed in the same vicinity, i.e. they are similarly surrounded by other nearby radiating elements and by metal partition walls whose function is described below.
In the case of a DCS and GSM dual band antenna, one feature of the wavelengths used facilitates the production of an antenna meeting the above two criteria. The average wavelength xcexDCS of the DCS band is approximately equal to half the average wavelength xcexGSM of the GSM band. It is therefore possible to produce an antenna having a periodic structure with the pitch for the DCS radiating elements equal to twice the pitch for the GSM radiating elements. Because of this feature, any GSM radiating element 12 is equidistant from two GSM radiating elements 12 and equidistant from two DCS radiating elements 14. Similarly, any DCS radiating element 14 is equidistant from two DCS radiating elements 14.
The symmetry in the disposition of the radiating elements of the two devices considerably reduces the consequences of radio frequency interference because each radiating element of the same device is affected by similar interference. The performance of a devicexe2x80x94for example its signal to noise ratioxe2x80x94is improved if the radiating elements of the device operate under similar conditions.
Coupling between radiating elements of the same device substantially reduces its performance. To reduce such coupling, the radiating elements are partitioned off by metal walls whose positions also determine various characteristics of the radiation of each device, for example the horizontal aperture. Thus walls 26 perpendicular to a longitudinal axis 27 of the antenna partition off the GSM radiating elements 12 within rectangular enclosures also defined by the longitudinal walls 27a and 27b of the chassis of the antenna. The walls 26 reduce the coupling between the GSM radiating elements 12, thereby increasing the gain of the GSM device.
The gain of the GSM device is a function of the distance between the lateral walls 27a and 27b and the GSM radiating elements 12 and of the height of the walls 27a and 27b. If the GSM radiating elements 12 are substantially equidistant from the partition walls 27a, 27b and 26, an optimum configuration is obtained enabling the GSM device to operate in accordance with transmission criteria imposed by operators. Furthermore, the operation of the GSM device is optimized in terms of the second criterion previously referred to, because all the radiating elements of the device are similarly partitioned off.
Furthermore, the walls 26 are also used conjointly with fixed walls 24 along the axis 27 of the antenna to partition off the DCS radiating elements 14. This partitioning determines operating characteristics of the DCS device, such as its horizontal aperture or its gain. Nevertheless, the GSM radiating elements 12 are also placed along the axis 27 of the antenna. Metal walls close to a radiating element disturb its operation. For this reason the longitudinal walls 24 have a chamfer 25 near the GSM radiating elements 12.
The DCS radiating elements 14 are partitioned off in pairs of radiating elements in rectangular enclosures formed by the walls 24, 26 and 27b. To limit coupling between the DCS radiating elements 14 of each pair, a wall 28 is placed perpendicularly to the axis 27 between the radiating elements 14 of the pairs. Each wall 28 is equidistant from the two DCS radiating elements 14 separated in this way. Accordingly, these walls 28 are in the vicinity of a GSM radiating element 12 equidistant from the said two DCS radiating elements. The walls 28 therefore interfere with the GSM radiating elements 12 in the same way as the walls 24, because of the proximity of a partition wall to the GSM radiating elements 12. For this reason the walls 28 have a length which is less than the width of the enclosures partitioning off the DCS radiating elements 14. Moreover, the height of the walls 28 decreases as they approach the GSM radiating elements 12.
This decreasing profile represents a compromise between partitioning off the DCS radiating elements 14 and the disturbance of the GSM radiating elements 12 caused by these walls. Reducing the height of the wall 28 in the vicinity of the GSM radiating elements 12 reduces interference between the wall and the GSM radiating elements 12. The DCS radiating elements 14 are then substantially equidistant from the walls 24, 26, 27b and 28. As with the GSM radiating elements 12, this disposition is a result of optimizing the performance of the DCS device. Moreover, the partitioning being similar for all the DCS radiating elements 14, the interference suffered by all DCS radiating elements 14 is similar, thereby optimizing the operation of the DCS device.
Producing a dual band antenna made up of radiating elements specific to each transmission system therefore necessitates many compromises and artifices to enable correct operation of each device. Moreover, because the average wavelength xcexDCS of the DCS band is approximately equal to half the average wavelength xcexGSM of the GSM band, it is possible to situate the set of DCS and GSM radiating elements periodically along the axis of the antenna with an optimum distance between them.
The object of the invention is to propose a triple band antenna, for example a GSM/DCS/UMTS antenna, which operates satisfactorily even though the average wavelength of at least one band is not a multiple or sub-multiple of the average wavelengths of the other two bands.
The invention provides a radio antenna, in particular for use in cellular telecommunications, including first, second and third radiating elements adapted to operate in three respective different frequency bands, wherein the structure of the antenna is periodic along a longitudinal axis and, in each module of the structure, a first radiating element is placed at the center of a quadrilateral, two adjoining vertices of which are each occupied by one of the second radiating elements and the other two vertices of which are each occupied by one of the third radiating elements. Accordingly, the operation of each type of radiating element is optimized because each element of the same type is surrounded by a similar immediate vicinity, even though the average wavelength of at least one of the bands is not a multiple or sub-multiple of the average wavelength of the other two bands.
In a preferred embodiment, respective radiating elements are aligned in three rows parallel to the longitudinal axis of the antenna and corresponding to respective bands.
In a preferred embodiment, two adjoining radiating elements adapted to operate in the same frequency band are separated by a distance of 0.95xc3x97xcexm, where xcexm represents the average wavelength of the frequency band.
In a preferred embodiment, in each module, the second radiating elements and the third radiating elements are placed in respective partitioned enclosures.
One particular embodiment of the antenna includes respective radiating elements adapted to operate in the DCS frequency band from 1 710 to 1 880 MHz, the GSM frequency band from 870 to 960 MHz and the UMTS frequency band from 1 900 to 2 170 MHz.
In a preferred embodiment, each module includes a GSM radiating element, a pair of UMTS radiating elements, and a pair of DCS radiating elements, and the two pairs of radiating elements define an approximate rectangle at the center of which the GSM radiating element is placed.
A triple band antenna of the above kind reduces installation, leasing and/or maintenance costs for the operator of a network wishing to introduce radiating elements for a new communication systemxe2x80x94for example the UMTSxe2x80x94into its network at the same time as continuing to use systems already in use.
Moreover, the above kind of antenna has the advantage over a wideband antenna of using independent radiating elements for each telecommunication system. An operator equipped with the above type of antenna can therefore vary the coverage area of one of the telecommunication systems without modifying the coverage areas of the other systems using the antenna. The transmission coverage area of a device is varied by varying the signals feeding the device. It must be pointed out that a wideband antenna cannot effect this kind of modification, the device operating for each of the communication systems being the same.
Other features and advantages of the invention will become apparent from the description of some embodiments thereof, which description is given by way of non-limiting example and with reference to the accompanying drawings.